Yes, but you can reliable use their service with a generic email client, specially on iOS for instance. The bridge doesn’t even provide everything a IMAP server does and there’s isn’t a way to get get calendars and contacts.
That bridge and the fact they don’t use generic IMAP/SMTP/CardDav/CalDav is a form of vendor lock-in. Other providers are also capable of encrypting email with PGP on a open manner and still use those generic protocols.